Warning! The following contains spoilers for The Flash episode "Mother." Read at your own risk!
The Flash finally ended its Mirror Master (or Monarch?) arc, and while the episode brought lots of unexpected moments like the return of the original Harrison Wells, the problematic Ralph Dibny, and Iris West temporarily gaining powers, no twist could hold a candle to that ending scene. Granted, we're still not entirely sure what that final scene meant, though a current theory is exciting.
To recap what happened, viewers of The Flash were taken back to the previous scene in which Iris and Barry's love brought back the original Speed Force. As sweet and slightly corny as that was, it's what happened afterward that Team Flash didn't see that has viewers talking. The Speed Force energy emitted from a lightning rod atop S.T.A.R. Labs was met by lightning that was green, yellow, red, and blue all from different directions.
The Flash offered no explanation for the moment in the episode, though some have already noticed that the colors correspond to different types of Forces in The Flash universe. Yes, the Speed Force (Yellow) is not the only Force in this fictional universe. There's also the Sage Force (Blue), the Strength Force (Red), and the Still Force (Green). In the comics, all these forces represent energy needed to maintain a multiverse and reality and emanate from Barry Allen himself.
With that said, these forces can exist in others and grant them powers in a way similar to how Barry gained super speed from the Speed Force. The Sage Force enhances the minds of those bound to it, Strength Force enhances the strength, and the Still Force is inertia. The Still Force is the counter to the Speed Force, due to its reliance on a lack of movement.
The concept itself isn't that hard to understand, and one might say that it's similar in some ways to the checks and balances between colors in Green Lantern stories from DC Comics mixed with some lore of Marvel's Infinity Stones. My assumption is the next batch of episodes may address these new Forces in some way, assuming that's what's going on.
Another possibility is new speedsters with different colored lightning, which is something The Flash has done in the past. The show has featured red, yellow, and blue lightning, but the green one is new. With that said, there are Flash characters in DC Comics who have green lightning, so maybe there are just more speedsters headed to the show. There was a tease for Godspeed's return in the Season 7 trailer a while back, so perhaps this event brings him back to town.
The bottom line is this is all just speculation, and we don't know for sure what The Flash has planned for the rest of the season. It does seem that something big is on the way to The Flash, and hopefully, it will be as exciting going forward as it has been in the past.